In order to measure the viscosity of a hydrocarbon reservoir fluid, a sample of the reservoir fluid is taken and analysed under reservoir pressure and temperature. A brief description of the way in which a PVT analysis is carried out is given in section 3 of the book Contributions in Petroleum Geology and Engineering, Volume 5, Properties of Oils and Natural Gases, K. S. Pederson et al, 1989. Such an analysis can be very accurate, however it takes a long time to be completed.
It is of great importance to know the viscosity of the reservoir fluid as soon as possible, preferably directly after a well has been drilled. Because, then there is still a possibility to adjust the design of the production and surface equipment to take into account the actual viscosity.
There are analysis tools, such as the modular dynamics formation tests from Schlumberger, the repeat dynamic tester from Halliburton and the reservoir characterization instrument from Western Atlas that are provided with an optical fluid analyser. Such an analyser operates by subjecting the fluid to be analysed to an absorption spectroscopy in the visible and near infrared ranges. The analyser measures the transmittance (which is the ratio of transmitted light energy to incident light energy) at different wavelengths. The output of the analyser is the optical density spectrum (which is the optical density, log(1/transmittance), as a function of wavelength).
Reference is made to SPE Paper 39093, Determination of producible hydrocarbon type and oil quality in wells drilled with synthetic oil-based muds, M. N. Hashem et al, 1997. In this paper it is disclosed that there is a correlation between the output of the analyser and the API gravity and between the output of the analyser and the gas-oil ratio.
Reference is further made to SPE paper 63252, Determination of hydrocarbon properties by optical analysis during wireline fluid sampling, A. van Dusen et al, 2000. This paper discloses that there is a correlation between the output of the analyser and some of the PVT properties, where PVT is an acronym used to refer to pressure, volume and temperature. According to this publication, density, saturation pressure, oil compressibility, formation volume factor and gas-oil ratio gave a good correlation, and that weaker correlations were found with other PVT properties.